I was trying to add the flavor of the retrieve method that reads into an existing instance, when I realized that there are just too many orthogonal axes here to rely on overloaded methods.
That calls for a builder pattern, which we already have --- it's called Poster, but it can actually already handle GET and other HTTP requests.
So I'm retiring the retrieveXYZ methods and moving the code into Poster. This is the first step.
In various places of the GitHub API, there's often a full object and
then there's a shallow object. I think the client API would be a lot
easier to use if a single class represents both and retrieve additional
fields on the fly as needed.
